Yemeni security forces have clashed with thousands of opposition fighters preparing to enter the capital, Sana'a, to reinforce fellow tribesmen loyal to Sheikh Sadeq al-Ahmar. The rising chaos is pushing the conflict closer to an all-out civil war.

Witnesses said Thursday that government troops, including elite Republican Guard units, confronted the tribal fighters at a key checkpoint north of the capital. Sana'a is split, with Yemeni security forces holding the southern part of the city against tribesmen and renegade military units in the north. Dozens of people have been killed in the most recent fighting.

The clashes threatened to close the Sana'a airport, which lies 10 kilometers north of the city. The streets of the capital are now largely deserted, as thousands of residents flee to safer ground.

U.S. envoy John Brennan, President Obama's top counterterrorism adviser, left Saudi Arabia for the United Arab Emirates Thursday to continue talks on Yemen. He is seeking the help of the two countries' leaders to pressure Yemeni President Ali Abdullah Saleh to accept a deal brokered by regional powers that would secure a peaceful end to his nearly 33-year rule.

Yemen is engulfed by multiple conflicts, with the street battles raging in Sana'a, popular unrest by anti-government demonstrators throughout the country and fighting against Islamist militants who have seized the southern city of Zinjibar.

The fighting in Sana'a broke out last week when pro-Saleh forces moved against the al-Ahmar compound in Hasaba, a district of the capital. In March, the al-Ahmar family had announced that the Hashid confederation – the country's most powerful tribal alliance – would back the protest movement, but its armed fighters had avoided clashes with Mr. Saleh's forces.

In the southern city of Taiz, clashes were reported for the first time Thursday between government forces and protesters calling for an end to Mr. Saleh's rule. At least 25 people have died in the violence in Taiz in the past few days. The city remains in a state of lockdown.

Yemen's unrest has caused a number of countries to close or scale back their diplomatic missions. Kuwait said Wednesday it had evacuated its embassy staff, joining Italy, Qatar and Germany in suspending operations in Yemen. The U.S. last week moved most non-essential staff out of the country.